During welding, contamination and/or oxidation of the molten, solidifying and cooling metal by the atmosphere must usually be avoided. In some types of welding processes, this is accomplished by the use of fluxes and/or slags, while in other welding processes this is accomplished by welding in a vacuum. Another very common method of protecting the hot and reactive weld metal is to surround it with inert shielding gases until it cools. Welding processes which use such inert gas protection are generally well known in the art and include, but are not limited to, gas metal arc welding (GMAW), gas tungsten arc welding (GTAW) and plasma arc welding (PAW).
In general, because of its relatively low cost and extreme inertness, argon is often used with these gas shielded welding processes as the principal shielding gas. This is particularly true with the gas tungsten arc welding process, where 100% argon is nearly always recommended. While the gas composition has only minor effects when gas tungsten arc welding many common metals, the number of welding transfer modes possible with the gas metal arc welding process have necessitated a wide range of mixtures of shielding gases to accommodate and optimize the various modes of metal transfer.
While generally these inert shielding gases are primarily designed to exclude atmospheric oxygen from the weld zone, small controlled amounts of active (oxidizing) gases, such as carbon dioxide and/or oxygen, are often added. When these active gases are blended with inert shielding gases, usually argon and/or helium, unique conditions are known to be produced during gas metal arc welding.
A variety of known GMAW shielding gases, some of which contain controlled amounts of oxygen and/or carbon dioxide, are discussed below. Many of these gases have received U.S. patent coverage as noted. Generally, in reviewing the literature concerning these GMAW shielding gas mixtures, a specific weld metal transfer mode is discussed or recommended. That is, the gas mixture has been blended to optimize welding in one specific GMAW transfer mode. As apparent from even a cursory review of the prior art and the patents, there are contradictory opinions relating to the proportions of carbon dioxide, helium and argon in shielding gas mixtures.
U.S. Pat. No. 2,591,926 discloses an argon-base gas containing about 20 to 60% (preferably 40-50%) helium suitable for short-arc welding of aluminum, stainless steels, or nickel alloys.
U.S. Pat. No. 2,753,427 discloses welding gases containing a range of 40 to 80% helium, 3 to 10% carbon dioxide and balance argon. The composition preferably contains 10 to 57% argon, 40 to 70% helium and 3 to 5% carbon dioxide.
U.S. Pat. No. 3,042,484 discloses a gas containing 1 to 30% argon in a carbon dioxide base suitable for protecting molten metal from oxidation.
U.S. Pat. No. 3,139,506 discloses a gas containing 20-70% CO.sub.2, 1-15% O.sub.2 in an argon or helium base.
U.S. Pat. No. 4,463,243 discloses shielding gas compositions containing 25 to 60% helium, 3 to 10% carbon dioxide, 40 to 70% argon and 0.1 to 1% oxygen.
The invention evidently resides in a shielding gas with "uniquely proportioned four-gas mixture" that provides unexpected improvements.
U.S. Pat. No. 4,749,841 discloses a range of 16 to 25% helium, 1 to 4% carbon dioxide and 71 to 83% argon. The composition preferably contains about 20% helium and about 3% carbon dioxide and specifically teaches against less than 1% carbon dioxide. The proportions of gases are blended within a narrow range thereby providing a "sufficiently precise control of the minor constituents" to avoid the harmful effects of carbon dioxide contents below about 1%.
U.S. Pat. No. 4,857,692 discloses a shielding gas for use in spray mode gas metal arc welding. This shielding gas contains 3 to 8% carbon dioxide, 30 to 40% argon and balance helium. The only specific example of a shielding gas composition contained 61% helium, 35% argon and 4% carbon dioxide.
U.S. Pat. No. 4,871,898 discloses a constant arc voltage, gas metal arc welding process. The shielding gas mixture (sold commercially under the trademark HELISTAR by Union Carbide Corporation) contains 2 to 12% and preferably 8 to 12% carbon dioxide, 20 to 45% helium and the balance argon. The only specific example of a shielding gas contained 8% carbon dioxide. HELISTAR gas is reported to be useful for spray and pulse-spray arc welding of carbon and low-alloy steels. HELISTAR SS gas is similar (30% He, 1% CO.sub.2 in argon) and is reported to be useful for stainless steels.
Other commercially available shielding gases include: Grade A-1025 which is a shielding gas containing argon and carbon dioxide in a helium base and is specifically blended and recommended for GMAW short circuit transfer mode welding with stainless steel.
Grades 99-1 and 98-2 are argon based shielding gases, containing 1% or 2% oxygen, respectively, which are routinely recommended for spray transfer welding of stainless steel.
In each of the above references, it appears that the shielding gas is designed to perform in some specific transfer mode of gas metal arc welding and often takes into account some specific alloy system to be joined.
It is also known in the art that the melting and welding of the Nickel-based and Cobalt-based Alloys requires special care.
For many years, argon and/or argon-helium mixtures have been recommended and used as shielding gases for welding the superalloys, for example the nickel-based HAYNES.RTM. and HASTELLOY.RTM. alloys. A shielding gas containing 75% argon and 25% helium (identified herein as "75-25" and apparently first disclosed in U.S. Pat. No. 2,591,926) has routinely been recommended during short circuit transfer mode welding while 100% argon has been recommended routinely for spray transfer mode gas metal arc welding. HAYNES and HASTELLOY are registered trademarks of Haynes International, Inc.
When using the 75-25 shielding gas in the short circuit mode, bright-shiny welds are produced. In addition, excellent cleaning action is noted along the edge of the arc. As a result, only minimal wire brushing is required between weld passes. However, with this shielding gas, the arc is erratic and unstable which leads to excessive spatter and poor bead appearance. In addition, during dissimilar welding between carbon steel and nickel-based alloys, such as HASTELLOY C-22 alloy, the arc is highly unstable, does not strike off easily and produces excessive spatter.
The solution to this problem, in the short circuit mode, is to use a shielding which contains carbon dioxide. One common gas which has been used with the nickel-based alloys is grade A-1025. This gas contains 2.5% carbon dioxide with small amounts of argon (about 71/2%) in a helium base. This gas will produce an exceptionally stable arc, with little or no weld spatter. In addition, during dissimilar welding, the arc will strike-off immediately producing a very stable arc. The drawback to this gas, when welding with nickel-based alloys, is the formation of a very heavy black oxide scale on the weld metal surface. Such an oxidized surface requires considerable interpass grinding and/or heavy wire brushing when making multipass welds. Because the molten metal surface is highly oxidized, welding travel speeds must be low, so as to allow enough time for the weld metal to flow outwardly and tie into the adjacent base material.
As a result of this oxidized condition, considerable economic penalty must be paid for lower production rates and higher labor costs. A second economic penalty must usually be paid as a result of the higher cost of a helium rich shielding gas.
A somewhat similar situation exists during spray transfer welding of the nickel-based alloys. With the use of 100% argon shielding gas, the arc tends to be unstable with considerable tendency for "arc-wander". The addition of helium is not known to improve arc stability under this mode of weld metal transfer. With such a situation, the welding operator has limited control of the process and less than optimum welds are often produced.
The addition of 1% or 2% oxygen to the argon shielding gas will greatly improve arc stability and reduce "arc-wander". Such a gas, as with the carbon dioxide bearing gases, produces an oxidized weld metal surface, which requires additional grinding between weld passes. Again, this improvement in welding characteristics results in an economic penalty due to the higher labor costs of interpass grinding.
Recent improvements in gas metal arc welding shielding gases suggest that a single gas can be used with several GMAW metal transfer modes when welding stainless steel. A typical example is HELISTAR SS gas which is reported to contain about 1% carbon dioxide, about 30% helium and balance argon. However, as with the other oxygen or carbon dioxide bearing shielding gases, surface oxidation of weld metal is severe with the nickel-based alloys, making interpass grinding an economic consideration.